Field of the Invention
The present invention relates to an organic light emitting device, and more particularly, to an organic light emitting device having a micro cavity emission structure in which emission efficiency and a service life are enhanced.
Discussion of the Related Art
To date, liquid crystal display (LCD) devices are being widely used as flat panel display devices. The LCD devices use a backlight as a separate light source, and have technical limitations in brightness and contrast. On the other hand, since organic light emitting devices self-emit light, the organic light emitting devices do not need a separate light source and have relatively better brightness, contrast, and viewing angle, and thus, interest in the organic light emitting devices is increasing. Also, since the organic light emitting devices do not use a backlight, the organic light emitting devices are manufactured to be light and thin, and have low power consumption and a fast response time.
The types of organic light emitting devices are categorized into a top emission type, a bottom emission type, and a dual emission type according to an emission direction of light. The organic light emitting devices are categorized into passive matrix organic light emitting devices and active matrix organic light emitting devices depending on a driving mode.
FIG. 1 is a diagram illustrating a red, green, and blue pixel structure of an organic light emitting device having a micro cavity structure of the related art. FIG. 1 illustrates a pixel structure of an active matrix organic light emitting device having a top emission type.
Referring to FIG. 1, the organic light emitting device includes an anode electrode 10, a cathode electrode 70, and an organic emission layer. The related art organic light emitting device has a structure in which the organic emission layer is formed between the cathode electrode 70 injecting an electron and the anode electrode 10 injecting a positive hole. A capping layer (CPL) 80 is formed on the cathode electrode 70.
In the micro cavity structure, the anode electrode 10 is formed as a reflective electrode, and the cathode electrode 70 is formed as a semi-transmissive electrode, thereby forming a micro cavity structure. An optical cavity is formed between the cathode electrode 70 and the anode electrode 10. The cathode electrode 70 transmits some (for example, 60% of all light) of light emitted from the organic emission layer, and the remaining light (for example, 40% of all light) which is not transmitted is reflected to cause constructive interference suitable for each wavelength, thereby enhancing emission efficiency.
The organic emission layer includes a hole injection layer (HIL) 20, a hole transport layer (HTL) 30, a plurality of emission layers (EMLs) 52, 54 and 56, an electron injection layer (EIL, not shown), and an electron transport layer (ETL) 60. In this case, the electron injection layer (EIL) may be omitted.
One unit pixel includes a red pixel Rp, a green pixel Gp, and a blue pixel Bp of three colors. The organic emission layer of the red pixel further includes a red HTL 42. The organic emission layer of the green pixel further includes a green HTL 44.
The red emission layer 52 of the red pixel Rp is formed between the ETL 60 and the red HTL 42. The green emission layer 54 of the green pixel Gp is formed between the ETL 60 and the green HTL 44. The blue emission layer 56 of the blue pixel Bp is formed between the ETL 60 and the HTL 30.
When an electron generated from the cathode electrode 70 and a positive hole generated from the anode electrode 10 are injected into the EMLs 52, 54 and 56, the injected electron and positive hole are combined to generate an exciton. The generated exciton is shifted from an excited state to a ground state to emit red light, green light, and blue light from the red EML 52, the green EML 54, and the blue EML 56.
Due to an emission structure and a material of the emission layer, the related art organic light emitting device has limitations in emission characteristic and a performance of a service life, and thus, a method has been proposed in which emission efficiency is enhanced by changing a fluorescent material, forming the emission layers 52, 54 and 56, to a phosphor material. However, the method has a problem in which power consumption increases in a case of increasing luminance Also, the method has a problem in which emission efficiency is lowered when a light emitting material is changed for securing a long service life.
As a resolution of a display device advances to a high resolution, the number of pixels per unit area increases, and high luminance is needed. However, due to an emission structure of an organic light emitting device, a luminance Cd of a unit area A is limited. Also, since a current increases for enhancing luminance, a reliability of a device is degraded, and consumption power increases.
Moreover, among pixels of three colors of the organic light emitting device, a blue pixel has a shorter service life than those of a red pixel and a green pixel, and in manufacturing a display panel having the pixels (red, green, and blue pixels) of three colors, a service life of the display panel cannot be ensured.
Therefore, in order to solve main causes that hinder a quality of a productivity of an organic light emitting device, it is required to overcome technical limitations in enhancing a service life and emission efficiency of a device and decreasing consumption power. Also, it is required to develop an organic light emitting device for reducing power consumption and enhancing emission efficiency and a service life of an emission layer.